Organic light-emitting diodes (OLEDs) are useful in a variety of consumer products, such as watches, telephones, lap-top computers, pagers, cellular phones, digital video cameras, DVD players, and calculators. Displays containing light-emitting diodes have numerous advantages over conventional liquid-crystal displays (LCDs). For example, OLED displays are thinner, consume less power, and are brighter than LCDs. Also, unlike LCDs, OLED displays are self-luminous and do not require backlighting. Furthermore, OLED displays have a wide viewing angle, even in bright light. As a result of these combined features, OLED displays are lighter in weight and take up less space than LCD displays.
OLEDs typically comprise a light-emitting element interposed between an anode and a cathode. The light-emitting element typically comprises a stack of thin organic layers comprising a hole-transport layer, an emissive layer, and an electron-transport layer. However, OLEDs can also contain additional layers, such as a hole-injection layer and an electron-injection layer. Furthermore, the emissive layer can contain a fluorescent dye or dopant to enhance the electroluminescent efficiency of the OLED and control color output.
Although a variety of organic polymers can be used to prepare the hole transport layer in an OLED, poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate), PDOT:PSS, is a preferred hole-transport material. OLEDs containing this material typically have a low turn-on voltage and high brightness. However, a hole-transport layer comprising PDOT:PSS has many limitations including low transparency, high acidity, susceptibility to electrochemical de-doping (migration of dopant from hole-transport layer) and electrochemical decomposition. Moreover, PDOT:PSS is insoluble in organic solvents and aqueous emulsions of the polymer, used to prepare the hole-transport layer, have limited stability. Consequently, there is a need for an OLED comprising a hole-transport layer that overcomes the aforementioned limitations.